In most integrated circuits certain points of the circuit are to remain at a fixed voltage when the circuit operates. For example, certain points of the circuit are frequently to be connected “to ground” or “to VDD”, which means that these points have a voltage equal to zero or five volts, for example, respectively. Such points present the drawback of being sensitive to electrostatic discharges which have the risk of damaging the integrated circuit. Moreover, such points cannot be tested because they have a fixed voltage even when the circuit is in the test mode. This brings in the disadvantage of diminishing a test cover ratio during a test step of the integrated circuit.
In order to mitigate these disadvantages, these points are replaced by cells which deliver a fixed output voltage when the integrated circuit is in the operation mode. These cells comprise a flipflop which may thus be integrated with a test chain during a test step. When the circuit is in the test mode, it is possible to make the output voltage of this cell vary. Consequently, by virtue of the cells, the points that have a fixed voltage in the operation mode may be tested in the test mode. So this permits to increase the test cover ratio. Moreover, such flipflops are not very sensitive to electrostatic discharges. This diminishes the risks of damaging the integrated circuit by such discharges.
The cell marketed by the applicants under reference CMOS18 is an example of such a cell. This cell comprises a flipflop which has two outputs and is activated by a clock. This flipflop receives a signal which indicates whether the circuit is in the test mode or in the operation mode. In the operation mode, when the clock is active, one of the outputs of the flipflop has a fixed positive voltage, the other a zero voltage. Consequently, while taking one of the two outputs of this flipflop as the output of the cell, this cell can replace ground or the voltage VDD at certain points of the integrated circuit.
A disadvantage of such a cell resides in the fact that for obtaining a fixed output voltage in the operation mode, the cell is constantly to be activated by a clock while the circuit is in the operation mode. This brings with it a considerable current consumption. In certain applications the current consumption is a critical data. This is the case notably in portable applications operating on a battery such as a portable telephone.